1. Field of the Invention
The present invention relates to a quinoxaline derivative, and a light emitting element, a light emitting device and an electronic appliance using the quinoxaline derivative.
2. Description of the Related Art
Organic compounds can have a wide variety of structures as compared with inorganic compounds, and have a possibility to provide materials with various functions by appropriate molecular design. Because of these advantages, photo electronics and electronics that use a functional organic material have been attracting attention in recent years.
As examples of electronic devices using an organic compound as a functional material, there are solar cells, light emitting elements, organic transistors, and the like. These devices utilize electrical properties and optical properties of the organic compound. In particular, the light emitting elements have been significantly developed.
It is considered that the light emission mechanism of a light emitting element is as follows: when a voltage is applied between a pair of electrodes with a light emitting layer interposed therebetween, electrons injected from the cathode and holes injected from the anode are recombined in the light emission center of the light emitting layer to form molecular excitons, and energy is released to emit light when the molecular excitons relax to the ground state. A singlet excited state and a triplet excited state are known as the excited states, and it is thought that light emission can be obtained through either of the excited states.
Such a light emitting element has a lot of problems that depend on the organic materials. In order to solve these problems, improvement of an element structure, development of a material, and the like have been carried out.
As the most basic structure of a light emitting element, the following structure is known: a hole transporting layer formed of an organic compound with hole transporting properties and an electron transporting light emitting layer formed of an organic compound with electron transporting properties are stacked to form a thin film with a total thickness of about 100 nm, and this thin film is interposed between electrodes (for example, see Non-Patent Document 1: C. W. Tang et al., Applied Physics Letters, vol. 51, No. 12, pp. 913-915 (1987)).
When a voltage is applied to the light emitting element described in Non-Patent Document 1, light emission can be obtained from an organic compound having light emitting and electron transporting properties.
Furthermore, in the light emitting element described in Non-Patent Document 1, functions of the thin film are appropriately separated in such a manner that the hole transporting layer transports holes while the electron transporting layer transports electrons and emits light. However, various interactions (for example, exciplex formation) occur at the interface of stacked layers, which may cause a change in emission spectrum or a decrease in emission efficiency.
In order to suppress the change in emission spectrum and the decrease in emission efficiency that are caused by the interaction at the interface, a light emitting element in which functions of the thin film are further distributed has been developed. For example, proposed has been a light emitting element having such a structure that a light emitting layer is sandwiched between a hole transporting layer and an electron transporting layer (for example, see Non-Patent Document 2: Chihaya Adachi et al., Japanese Journal of Applied Physics, vol. 27, No. 2, L269-L271 (1988)).
In the light emitting element described in Non-Patent Document 2, in order to more effectively suppress the interaction occurring at the interface, a light emitting layer is preferably formed using a bipolar organic compound that has both electron transporting properties and hole transporting properties.
However, most organic compounds are monopolar materials in which either hole transporting properties or electron transporting properties dominate.
Therefore, a bipolar organic compound having both electron transporting properties and hole transporting properties is needed to be developed.